Data storage memories are widely used in computers and control systems of various types. Computers and control processors ordinarily use electronic random-access memories (RAMs) to aid in performing their processes. Such electronic RAMs have the advantage of high operating speed, but their volume storage density is relatively low, and they are volatile, in that the data stored therein is lost when the system is deenergized. To save the data in a volatile RAM preparatory to deenergization, the data is ordinarily transferred to a rewriteable permanent medium such as magnetic disc or magnetic tape. Disc and tape media are capable of storing large amounts of data, but have substantial initial access time requirements to initially access or locate the data, and also have data transfer rates which are limited by the serial nature of the tape or the track on a disc. These different memory types constitute a hierarchy which lacks fast access time, high storage capacity members.
Three-dimensional (3D) optical storage RAMs have been described, in which light beams address data elements (datels, also known as voxels) within the volume of the memory material, for writing data thereto, and for reading. An article entitled Applications of Photochromic Polymer Films, by A. E. J. Wilson, published at pp 232-238 of Volume 15, 1984 issue of Phys. Technol., printed in Northern Ireland, describes photochromic materials generally, their applications to optical data recording, and also lists desirable aspects of an erasable reusable optical recording medium, which include (1) high sensitivity for writing and erasing, (2) high storage capacity in bits per cm.sup.2, (3) nondestructive readout, (4) lack of fatigue, which is the ability to be cycled repeatedly without losing its characteristics, (5) archival storage or persistence of more than 10 years, (6) no requirement for development of the image, and (7) low cost and ease of fabrication. Canadian patent application 2,037,059, filed Feb. 26, 1991 in the name of Daniels, and laid open Aug. 27, 1992, describes a system using liquid crystals as the memory material, which are stained with a dye. A slight electric field is applied across the memory. Writing is accomplished by a light beam, which heats the dye through which the beam passes, and the heat is transferred to the adjacent or contiguous liquid crystal datels, to allow them to change state under the influence of the electric field. In one embodiment of the Daniels memory, the heating is accomplished by multiple intersecting beams of light. Patent Cooperation Treaty (PCT) patent application WO93/02454, filed in the name of Strickler, and laid open Feb. 4, 1993, describes development of a three-dimensional optical memory in which a fluorescent dye is the storage medium, but which is undesirable because of photobleaching, and also describes an improved three-dimensional optical memory in which changes in the refractive index of a photopolymer are used for storage, and in which intersecting beams of light are used to detect inhomogeneities (regions of altered index of refraction) in the medium. An article entitled Three-Dimensional Optical Storage Memory, authored by D. A. Parthenopoulos et al., and published at pp 843-845 of the Aug. 25, 1989 issue of OE Reports, published by SPIE, the International Society for Optical Engineering, describes a three-dimensional optical memory based on volume storage in an amplitude-recording medium, specifically the photochromic molecule spirobenzopyran, which in a I (spiropyran) state absorbs visible light by a two-photon absorption process (simultaneous absorption of two visible-light photons, corresponding to energy in the ultraviolet or UV range), and when excited takes on a II (merocyanine) state. The I state may correspond to an unwritten (logic 0) state, so that writing involves application of ultraviolet-energy light to create a II state in the datel region. The II state absorbs light in the green-red region of the visible light spectrum, and emits red-shifted fluorescence when excited with green light. Thus, reading is accomplished by applying a beam of green light to the datel, and the red shift identifies the written (logic 1) state. The persistence of the II state, however, ranges from a few minutes under ordinary conditions to a few weeks when cooled. An article entitled Potentials of two-photon based 3-D optical memories for high performance computing, by Hunter et al., published at pp 2058-2066 of Applied Optics, Vol. 29, No.14, 10 May 1990, also discusses the use of spirobenzopyran. The text Electronic Materials From Silicon to Organics, edited by L. S. Miller et al., and published by Plenum Publishing Corporation, 1991, includes at pp 471-483 a chapter entitled Photochromics of the Future, authored by H. G. Heller, which notes that the main reason that organic photochromic materials have not been developed for commercial applications is the problem of fatigue, and which describes the properties of fulgides and heliotropic compounds. An article entitled Two Photon Three Dimensional Memory Hierarchy, by S. Esner et al., presented at the July, 1992 SPIE meeting at San Diego, describes the abovementioned hierarchy of memories for use in computers and processors, and also describes two-photon secondary storage (memory) systems which have the potential for millisecond access time and Tbit/sec data transfer rates, in which spirobenzopyran material in a 3-dimensional memory is written by intersecting beams of light, and in which an HCl component of the memory material provides permanent stability of the written form. However, permanent stability implies an inability to erase and re-write, or to overwrite.
It should be noted that the abovementioned different colors of light are established by their wavelengths, which range in the visible spectrum from about 400 to 700 nanometers (nm), and it is also noted that wavelength and frequency of light are inversely related by the velocity of propagation of light (C). The velocity of light is constant within a particular medium, but different media exhibit different values of C.
Improved memories are desired.